Switching mode power circuits are well known, for example switch mode power supplies and switch mode motor controllers. Such circuits usually include one or more electronic power switching devices, for example a field effect transistor (FET), a bipolar switching transistor, a triac and/or a silicon controlled rectifier (SCR). Increasingly, on account of their relatively faster switching speed enabling coincidental use of more compact magnetic components such as ferrite transformers, FETs are becoming increasingly employed in switch mode power circuits.
An important parameter for consideration when designing switch mode power circuits is hard switching amplitude; hard switching amplitude is defined as a voltage developed across a switching device at a moment whereat the device is driven into a conductive state, namely turned on.
U.S. Pat. No. 6,069,804 describes a multi-output, multi-directional power converter that has an input bi-directional switch and at least a first output bi-directional switch. Moreover, the converter further comprises a coupled inductor having an input winding and at least one output winding. The input winding is connected in series with an input voltage source and an input bi-directional switch implemented using FET technology. Each coupled inductor output winding is connected in series with a corresponding output voltage source, for example a capacitor, and its respective output bi-direction switch also implemented using FET technology. The converter additionally includes a clock circuit for providing first and second control signals, each signal having first and second states. The first and second signals are connected to the input and output switches respectively. Moreover, the first and second signals are arranged to be substantially mutually complementary with regard to their states.
The power converter is susceptible to being modified to include resonant transition controlling means for sensing currents in the input and output windings as well as output voltage and from such current sensing together with a measure of output voltage from the converter for adjusting a clocking frequency of the converter for enabling the converter to function in a resonant mode.
The converter is potentially expensive to implement on account of its clock circuit being coupled to both input and output sides of the coupled inductor, such connection requiring additional coupling transformers to be included for controlling the switches. Moreover, the converter does not utilize hard switching amplitude information as an aspect of its operation.
U.S. Pat. No. 6,433,491 describes a method of generating a signal corresponding to hard switching amplitude. The method concerns the use of a capacitive divider for sensing primary winding potential in a transformer-coupled device. The method involves temporally controlled resetting of the divider in conjunction with a sample-and-hold circuit for providing a direct indication of the hard switching magnitude. However, the method requires precise timing information and is directly associated with the primary winding which is potentially at relatively high potentials, for example as in mains-supplied SMPS. Thus, this U.S. patent is regarded as elucidating a non-optimal method of determining hard switching amplitude.
The inventor has appreciated that it is desirable, for example not only in the aforementioned method but also in the power controller and other similar types of switch mode circuits such as switch mode power supplies, to measure hard switching amplitude. For example, in a switch mode power supply (SMPS) system, switching losses occur if one or more power controlling switching devices therein are turned on, namely driven to a conductive state, whilst a non-zero potential is developed there across.
In some SMPS applications, hard switching is unavoidable and the hard switching amplitude is variable, for example in response to changing SMPS loading conditions. In such circumstances, it is often desirable to provide regulation to other components depending upon this amplitude, for example for providing circuit protection shutdown in an event of circuit overload. Moreover, timing information pertaining to occurrence of such hard switching is often not available or relatively expensive to obtain, for example on account of a need to include additional isolation components where mains electrical input supplies are involved. An example of such a SMPS application is a bi-directional flyback converter including a transformer with primary and secondary windings, the primary winding being connected to a primary FET switching device; preferably, the primary device is turned on, namely switched to a conducting state, whilst a voltage developed there across is almost of zero magnitude, namely the primary device is preferably subject to soft switching. There thereby arises a need to monitor the hard switching amplitude of the FET device, such monitoring conventionally being achieved by including a control loop implemented substantially around circuits associated with the secondary windings. Thus, the hard switching amplitude is conventionally monitored at a secondary region of the bi-directional converter by monitoring a voltage developed across one of its transformer windings. In such a configuration, precise switching timing information pertaining to the primary windings is not normally available at the secondary circuit unless additional potentially expensive components are included.
The inventor has appreciated that it is especially desirable to be able to determine hard switching amplitude in switch mode circuits including transformer-type components by monitoring a signal developed across a secondary winding of such transformer-type components without there being a need to generate precise temporal information, thereby potentially reducing the cost and complexity of such switch mode circuits.